The present invention relates to a magnetic recording material. More particularly, the invention relates to a magnetic tape having improved running performance and electromagnetic conversion characteristics even when it is made thin.
Heretofore, the common magnetic recording material has been a magnetic tape made of biaxially oriented polyethylene terephthalate film as the base film. Such a magnetic tape, however, has a disadvantage that it becomes poor in running performance and electromagnetic characteristics when the tape cassette is miniaturized or the base film is made thin to extend the recording time. On the other hand, the magnetic tape (as disclosed in U.S. Pat. Nos. 3,177,277 and 4,226,826) made of reinforced polyethylene terephthalate film as the base film has also a disadvantage that it is poor in running performance and electromagnetic conversion characteristics when the base film is thinner than 9 .mu.m it is used as video tape. In other words, magnetic tape made of polyethylene terephthalate film as the base film cannot meet the requirement for miniaturizing the tape cassette and thinning the tape for extended recording time.
Other polyesters than polyethylene terephthalate include polyethylene bis(phenoxy)ethane-4,4'-dicarboxylate (another name: polyethylene-1,2-diphenoxyethane-4,4'-dicarboxylate) as disclosed in British Pat. No. 1171685. According to this British patent, the polyester is capable of melt film forming and biaxial orientation and can be formed into film of high strength. However, this film is not satisfactory in running performance and electromagnetic conversion characteristics as the base film of the magnetic tape.
Japanese Patent Publication No. 49-1795 discloses another polyester, [polyalkylene-.alpha.,.beta.-bis(2-chlorophenoxy]ethane-4,4'-dicarboxylate ). In this patent, this polyester is described to be melt formable into fibre and film. And the fiber actually made from this polyester in examples is reported to have a high Young's modulus. However, this patent only suggests that this polyester might be melt formed to a film but no such film is mentioned in the examples of this patent.
The present inventors prepared base films having different properties from many polyesters including the above mentioned two, and investigated the running performance and electromagnetic characteristics of magnetic tapes made from the base films. As the result, it was found that magnetic tapes made from the base film which is made of a specific polyester and satisfies the specific parameters is specifically superior in running performance and electromagnetic conversion characteristics even when the base film is thin. The present inventors completed this invention based on these findings.